All Digital Subscriber Line (DSL) techniques are collectively referred to as the xDSL, which is a technique for high speed data transmission over a telephone twisted pair. In addition to the base band transmission DSL based upon the Integrated Services Digital Network (ISDN) and the like, the pass band transmission xDSL makes use of the frequency division multiplexing technique to make the xDSL and the Plain Old Telephone Service (POTS) coexist on the same twisted pair, where the xDSL occupies the high frequency band and the POTS occupies the base band part below 4 KHz. A system providing multiple accesses for xDSL signals may be referred to as a DSL Access Multiplexer (DSLAM).
As a transmission channel, the telephone twisted pair has a distortion-free information capacity which shall satisfy the Shannon channel capacity formula. The transmission capacity of the channel can be increased appropriately if the noise energy is reduced. The crosstalk, especially the crosstalk at a high frequency band, is a technical issue causing noise and has become a serious obstacle to improving the channel transmission capacity in some scenarios.
FIG. 1 schematically illustrates a principle diagram of a crosstalk. Because the xDSL adopts frequency division multiplexing for uplink and downlink channels, a near-end crosstalk may not influence the system performance considerably but a far-end crosstalk may influence seriously the transmission performance of lines. In FIG. 1, x1, x2 and x3 denote signal transmitting points, y1, y2 and y3 denote corresponding far-end signal receiving points, solid line arrows denote normal signal transmission, and dotted line arrows denote a crosstalk caused by a signal transmitting point to the receiving points corresponding to other signal transmitting points. As apparent from FIG. 1, signals to be transmitted at the points x2 and x3 are crosstalk sources for signals to be transmitted at the point x1, and naturally signals to be transmitted at the point x1 are crosstalk sources for signals to be transmitted at the points x2 and x3. Therefore, for clarity, a branch of signals to be transmitted is described as a reference object while regarding other signals as their crosstalk sources hereinafter. Such descriptions can be adaptive to respective branches of signals. Distinguishing names used for signals are merely for convenience, but not intended to differentiate the signals substantively.
In order to address the problem of the degraded channel performance due to the far-end crosstalk, a method of coordinated signal processing was proposed in the industry to cancel a far-end crosstalk among respective branches of signals by use of the feature of coordinated transmission and reception at the DSLAM end. At present, the signals are processed with a fixed filter in the frequency domain based upon such a principle that crosstalk cancellation calculations are performed on the premise that a channel transmission matrix has been pre-known. For coordinated reception of signals, this method frequency domain filters respective frequency points of received signals in accordance with the pre-known channel transmission matrix, and then estimates input channel signals in a general decision feedback equalization method. The essence of the method lies in that: because the channel transmission matrix is known, the relationship between crosstalk components in the received signals and a crosstalk source may be deduced, so that received signals corresponding to the crosstalk source can be used to approximately simulate the crosstalk source, thereby implementing a crosstalk cancellation at the coordinated receiver. On the other hand, for coordinated transmission of signals, the method is similar to that for coordinated reception, except that the signals are pre-coded in the frequency domain before transmission instead of processing the signals undergoing a crosstalk, to pre-cancel a crosstalk which may occur. Therefore, the receiver receives the signals from which the crosstalk has been cancelled.
The above method has a disadvantage in that the channel transmission matrix has to be pre-known, but it may be difficult to obtain the matrix accurately and conveniently. Moreover, the matrix per se features slow time-variation and may be susceptible to a transmission environmental factor. Consequently, the above solution may be difficult to implement in practice. Therefore, it is necessary to provide more techniques for noise cancellation for use in various scenarios.